One example of conventional planar light-emitting elements (hereinafter referred to as “light-emitting element”) is shown in FIG. 18. The light-emitting element 100 includes a base 101, an anode electrode 102, a light-emitting layer 103 and a cathode electrode 104, the latter three of which are laminated in that order on the base 101. An anode terminal portion 105 protrudes from the anode electrode 102 and a cathode terminal portion 106 protrudes from the cathode electrode 104. Electric power is fed from a power source 108 to the anode terminal portion 105 and the cathode terminal portion 106 via a constant current circuit 107.
In operation of the light-emitting element 100, the anode electrode 102 injects holes into the light-emitting layer 103 and the cathode electrode 104 injects electrons into the light-emitting layer 103, upon application of an electric voltage. The holes and electrons thus injected are combined together in the light-emitting layer 103 to generate excitons. The excitons make transition to a ground state, thus emitting light.
If electric power is normally fed to the anode terminal portion 105, the in-plane electric potential gradient of the anode electrode 102 with respect to the light-emitting layer 103 grows higher due to a high sheet resistance of the anode electrode 102, which leads to increased luminance variations.
In view of this, it may be thought to employ a light-emitting element 110 having a plurality of anode terminal portions 105a and 105b as illustrated in FIG. 19. If an electric current is supplied in parallel through these anode terminal portions 105a and 105b, it is possible to suppress occurrence of the in-plane electric potential gradient of the anode electrode 102 with respect to the light-emitting layer 103 and to reduce the luminance variations.
However, the peak value of the electric current flowing through the anode terminal portions 105a and 105b varies depending on the position and size of the anode terminal portions 105a and 105b. For that reason, it is difficult to completely prevent occurrence of the electric potential gradient.
As a solution to this problem, it may be contemplated to employ a light-emitting element 110 to which variable resistors 111a and 111b are connected as shown in FIG. 20. The variable resistors 111a and 111b are connected between the anode terminal portions 105a and 105b and the constant current circuit 107, respectively. The resistance between the anode electrode 102 and the power source 108 can be matched by adjusting the resistance values of the variable resistors 111a and 111b. This makes it possible to suppress occurrence of the electric potential gradient in the anode electrode 102.
However, the resistances are varied from electrode to electrode due to a temperature gradient in the anode electrode 102 or the light-emitting element 110 caused by the installation state and direction of the light-emitting element 110. For that reason, it is difficult to equalize the current values supplied to the anode terminal portions 105a and 105b. 
It may also be contemplated to employ a light-emitting element 110 to which two constant current circuits 107a and 107b are connected as shown in FIG. 21. This makes it possible for the constant current circuits 107a and 107b to equalize the peak values of the current flowing through the anode terminal portions 105a and 105b. As a result, it is possible to suppress occurrence of the electric potential gradient when emitting the light. However, the power feeding circuit becomes complicated because there is a need to provide the anode terminal portions 105a and 105b in plural numbers.
It is known that the light emission characteristics of the light-emitting element vary depending on the composition of a material of which the light-emitting layer is made. This means that it is possible not only to adjust the emission color of the light-emitting element but also to increase the light emission efficiency and the lifespan thereof by properly setting the composition of the material of the light-emitting layer. Further, adjustment of the color of a light emission surface is very useful in the field of light sources. Nevertheless, it is impossible to adjust the color of the light emission surface because the composition of the material of the light-emitting layer cannot be changed after the light-emitting element has been fabricated.
In view of this, it may be contemplated that the adjustment of color of the light emission surface is not performed with a single light-emitting element but by changing the light emission intensity of individual light-emitting elements in a planar illumination device, e.g., a display device, in which a plurality of light-emitting elements differing in emission colors, e.g., red (R), green (G), blue (B) or the like is arranged side by side. However, this is not desirable because the light-emitting elements need to be made in a small size in the planar illumination device, which makes the planar illumination device structurally complicated.
There is also known a planar illumination device in which the light emission color can be arbitrarily changed by independently driving a plurality of laminated planar light-emitting elements each having transparent anode and cathode electrodes (see, e.g., JP2002-260859A and JP2003-288995A). Although such planar illumination device is capable of changing the color of a light emission surface thereof, the thickness thereof is increased because the light-emitting elements are overlapped one above another.